The present invention relates generally to spacecraft orbit control systems and methods and, more particularly, to a spacecraft orbit control system and method which process spacecraft position data to meet the spacecraft attitude sensing needs for long duration, low thrust orbit raising burns.
Traditional spacecraft orbit raising maneuvers employ high thrust thrusters burning for short durations. Precise spacecraft attitude and orbit information is vital to ensure obtaining the desired change in spacecraft orbit and the efficient use of the high thrust thrusters. While attitude can be maintained during the burn, the orbital trajectory is essentially open loop. Unfortunately, the attitude determination sensors on board the spacecraft often do not work during orbit raising for such reasons as the spacecraft being too close to earth, a spinning transfer orbit, obscured line of sight due to stowed configuration of the spacecraft, etc. Alternatively, on some occasions, the required attitude accuracy during the orbit raising exceeds that needed on-station. In either case, the orbit raising sensor requirements add to the cost and complexity of the attitude determination system, often requiring dedicated sensors and sensor processing.
Ideally, what is needed is a spacecraft orbit control system and method for spacecraft orbit raising in which the absolute attitude determination accuracy requirements are dramatically relaxed relative to traditional spacecraft orbit raising techniques.
The foregoing and other advantages are provided by a method and apparatus for a spacecraft orbit control system and method which process spacecraft position data to meet the spacecraft attitude sensing needs for long duration, low thrust orbit raising burns. A control system for controlling the orbit of a spacecraft during orbit raising is provided. The control system includes an actual spacecraft position sensor that generates an actual position signal. A trajectory provider that includes an orbital determination provider, a memory, a first comparator, and a trajectory calculator is also included in the control system. The orbital determination provider generates an orbital model. The memory contains information of a desired orbit. The first comparator generates an orbital error signal in response to the desired orbit and the orbital model. The trajectory calculator determines a trajectory in response to the orbital error signal. A desired spacecraft position provider generates a desired position signal, which represents a desired position along the trajectory. The second comparator generates a positional error signal in response to the actual position signal and said desired position signal. A spacecraft attitude control system adjusts attitude of the spacecraft in response to the positional error signal.
Accordingly, it is an advantage of the present invention to provide a spacecraft orbit control system and method which processes spacecraft position data to meet the spacecraft attitude sensing needs for long duration, low thrust orbit raising burns.
Another advantage of the present invention to provide a spacecraft orbit control system and method in which the actual spacecraft position is compared with the desired spacecraft position during long duration, low thrust orbit raising burns to generate an error signal indicative of a spacecraft position error for adjusting the attitude of the spacecraft and eliminating the spacecraft position error such that the actual spacecraft position corresponds with the desired spacecraft position.
Furthermore, another advantage of the present invention is that it provides a spacecraft orbit control system and method for controlling the orbit raising of a spacecraft during long duration, low thrust orbit raising burns by closing an actual spacecraft position sensing loop around the desired spacecraft position such that spacecraft attitude is within the closed position loop and, hence, relatively low accuracy spacecraft attitude sensing yields acceptable orbit raising performance.
The advantages accruing to the present invention are numerous. For instance, the present invention is a cost effective alternative when using long duration burns with low thrust thrusters augmented by a periodic spacecraft position measurement. Because the spacecraft attitude is inside the closed position loop and is adjusted as necessary to keep the trajectory of the spacecraft on track, the absolute spacecraft attitude determination accuracy requirements are relaxed relative to the prior art.
These and other features, aspects, and embodiments of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings.